1895. ] Present Problems in the Cactacee. 217 
bly water-holding, as is the mucilage, but this is proven in 
neither case. To the mucilage, Stahl?* assigns a function of 
protection against animals, which its abundance in some non- 
xerophilous forms seem to sustain. The ‘‘Nebenzellen” of 
the stomata are certainly a xerophilous character, but in what 
way are they? Probably nowhere else in nature, though it is 
a xerophilous character, are the rings and spirals better de- 
veloped in tracheids than here—they are particularly superb 
in Leuchtenbergia, where as elsewhere they often form 
gland-like masses along or at the ends of fibro-vascular bun- 
dies. What is the use of the spirals in this water-holding 
family? Can it be that the spiral in a tracheid is associated 
with the ho/ding of water, and that it acts by presenting a 
larger surface and hence stronger capillary attraction for the 
water? And are the gland-like tracheid masses really water 
Storers, and if so why are they better than the ordinary pith 
or cortex cells? The entire internal anatomy of Leuchten- 
bergia is most beautiful and interesting and the study of its 
development will prove of great interest. Pits between 
water cells are particularly fine and show thickened plates at 
the contact-walls. The hairs show many fine markings and 
are recommended to those interested in the structure of cell- 
walls. A micro-chemical study of the tissues is needed, for 
I have frequently found that the reactions to reagents are 
not such as our suppositions as to the nature of the walls 
lead us to expect. The crystals are excessively abundant, 
doubtless in part because of the very slow growth of desert 
Cactacez and the lack of falling parts, leaves and bark. In 
Certain cases, as in Q. arborescens, use seems to be made of 
them for they are deposited in a close single layer, appar- 
ently as a sort of armor, just under the epidermis, and per- 
haps hinder the ravages of snails, or other small animals. 
But do snails occur on the desert and try to eat the Cactacex? 
This is but one of many examples of what seem to be the con- 
verting of a disadvantage to use; probably many of the special 
secretions, substances once purely excretions but converted 
to protective purposes, are of this origin. ee 
c e Cactacez show great ‘'‘vitality,” not only in withstand- 
ing bad treatment, but in rooting freely from almost any part, 
as from the leaves in Peireskia, the tubercles in Mamillaria, 
etc. Perhaps the possibility of free grafting is another phase 
reece cn ee OLA 
**Pflanzen und Schnecken, Jena. 1889. 
